This invention relates to the production of fine stable latexes of block copolymers of vinyl aromatic hydrocarbons and conjugated dienes. More specifically, it relates to the production of such latexes which have fine particle sizes and low surfactant to polymer ratios by using appropriate blends of nonionic surfactants.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise blocks of these different monomers such as configurations which are linear, radial or star, i.e. many arms radiating from a central core. The proportion of thermoplastic blocks to elastomeric blocks and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics.
It has been found advantageous to prepare latexes of these polymers in order to obtain products that can be formed into coatings and adhesive films. Preparation of such latexes is well known to those skilled in the art (see U.S. Pat. Nos. 3,360,599, 3,238,173 and 3,726,824). Latexes of polyvinyl aromatic--polydiene block copolymers are conventionally made by (1) dissolving the block copolymer in an organic solvent or solvents to form a polymer cement, (2) emulsifying the polymer cement in an aqueous solution containing one or more surfactants, and (3) stripping the emulsion of organic and excess aqueous liquids to form a stable, suitably concentrated latex. Polymer cements are often available directly from the polymerization reactor.
Prior art block copolymer cement solutions that are to be emulsified typically contain from 5 to 30 weight percent polymer. It is advantageous for the polymer concentration in the cement to be as high as possible. Increasing the polymer concentration in the cement reduces the amount of solvent and excess water that must be removed following emulsification. It also reduces the surfactant to polymer ratio in the final latex. Surfactants are usually present such that the total concentration of surfactants ranges from about 0.5 to more than 50 parts per hundred rubber (phr), where the rubber is the block copolymer and phr is based on 100 parts by weight of the rubber. A high surfactant concentration can negatively impact adhesive properties of coatings and adhesives. For these reasons, it would be advantageous to be able to have a high solids content latex.
Ionic surfactants have been used as emulsifiers for block copolymer cement solutions. Very good results in terms of particle size are sometimes achieved using such ionic surfactants. However, latexes made with ionic surfactants are generally only able to achieve a solids content of 40 to 45 percent. Further, such latexes have a problem with freeze-thaw stability, i.e. after the latex is frozen, it does not return to its original state and is not redispersible. Such latexes are also sometimes pH sensitive in that they may coagulate in lower or higher pH water. Further, such latexes are sometimes very sensitive to the hardness of the water in which they are emulsified. They do, however, exhibit good long term stability. It would be advantageous to have a latex which exhibited long term stability and was able to achieve a higher solids content and exhibit good freeze-thaw stability, low pH sensitivity and low water hardness sensitivity.
Fine polymer cement emulsions are necessary in order to make fine latexes. Fine particle size latexes are advantageous because particles remain stable and dispersed with less surfactant. Large particles tend to either settle or cream depending on the density of the cement relative to the aqueous phase. It is also known that small particle sizes in latexes can enhance coating and adhesive performance. Thus, it is highly advantageous to be able to produce a stable polymer cement emulsion with a small average drop size, for example, one micron or less, without the formation of appreciable amounts of multiple emulsions.